Silencer

ABSTRACT

In an expansion chamber silencer, such as an exhaust silencer for internal combustion engines, the inlet pipe is extended into the expansion chamber and is terminated by a cross plate diffuser converting the fluid flow through the inlet pipe into a curtain flow entering the expansion chamber in a direction that is substantially radial to the inlet pipe direction.

United States Patent 11 Frederiksen 1 1 SILENCER I75] Inventor EyvindFrederiksen, Soborg,

Denmark [731 Assignee: A/S Silentor, \"cdbaek, Denmark [ZZI Filed: Jan.14, 1974 [21] Appl. No: 433.423

Related US. Application Data [631 Continuation of Ser No 31193115, Not34, 1972.

abandoned 1301 Foreign Application Priority Data Nov 14 1971 Denmark 15760/71 [52] US. Cl v1 181/57; 181/69 1511 Int. Cl.'-' FOIN 1/08 158]Field of Search. 181/49 50, 57, 69

1561| References Cited UNITED STATES PATENTS 2Z7 3()Z 5/1880 Richards 1v 1 v 1 11/57 756,203 4/1904 Barthel v v H 181/57 [451 Aug. 5, 197515131907 10/1922 Cramer 1, 181/50 1,756 916 4/1930 Stranahan. 1. 181/692031884 4/1936 Day 1 1 .1 181/511 2,144,631 1/1939 Kurthm v. 181/502.239.549 4/1941 Chipley 181/57 1667940 2/1954 Gallihugh 1 U 181/57238L852 4/1959 Morrish 181/57 2931707 5/1960 Ernst 181/51) 3,016,9721/1962 Dugas 181/57 Primar Examiner-Stephen .1. Tomsky AssistantE.\'aminerVit W, Miska Attorney, Agent, or Firm-Br0wne, Beveridge.DeGrandi & Kline [5 7] ABSTRACT In an expansion chamber silencer, suchas an exhaust silencer for internal combustion engines. the inlet pipeis extended into the expansion chamber and is terminated by a crossplate diffuser converting the fluid flow through the inlet pipe into acurtain flow entering the expansion chamber in a direction that issubstantially radial to the inlet pipe direction.

11 Claims, 6 Drawing Figures PATENTED AUG 5 I975 SHEET F/G Zn SILENCERThis is a continuation of application Ser. No. 309,305, filed Nov. 24,1972 now abandoned.

The invention relates to a sllincer of the expansion chamber type forreducing noise propagation by the flow of gaseous fluids through tubularducts, e.g. exhaust pipes for internal combustion engines.

The bounding wall of an expansion chamber silencer comprises an aperturein communication with an inlet duct and an aperture in connection withan outlet duct, and the transverse dimension and the length of thechamber are larger than the diameter of the inlet and outlet ducts. Theprinciple permits several apertures for inlet as well as for outletducts, but in this case this will be disregarded for the sake ofsimplicity.

The effect of an expansion chamber silencer consists in the fact thatchamber and outlet duct detain a greater or smaller part of the noisearriving from the inlet duct.

With regard to the effect, the outlet duct forms an integral part of thesilencer. With regard to the construction, chamber and outlet duct mayform separate parts which are intended for being assembled by connectingbranches, flanges or the like.

The reduced noise issuing from the silencer depends generally on thenoise which from the inlet duct is induced in the chamber, and on thecapability of the chamber and the outlet duct of detaining this noise.

The function of the silencer may be further explained in connection withthe conventional design in which the expansion chamber consists of acylindrical casing with two flat end plates for the external and coaxialconnection of the inlet duct at one end plate and the outlet duct at theother end plate.

The noise which from the inlet duct is induced in the chamber depends onthe degree to which the noise content of arriving sound waves isreflected from the crosssectional transition between inlet duct andchamber, and on the formation of flow eddies caused by the fluid whenflowing into the chamber.

The reflection of the sound waves, arriving through the inlet duct, atthe said cross-sectional transition depends on several parameters,including particularly the pitch or frequency of the sound waves andthereby their wave length, the flow velocity in the duct as well as theproportion between the cross-sectional areas of the chamber and theinlet duct.

Low-frequency sound waves having wave lengths considerably larger thanthe transverse dimension of the chamber will to a high degree bereflected from the cross-sectional transition, the better the larger thecross-section of the chamber in proportion to the crosssection of theduct.

High-frequency sound waves having wave lengths of the same orderofmagnitude as the cross-section of the inlet duct, or less, will onlyto a negligible degree or not at all be reflected from thecross-sectional transition, even in the case that the cross-section ofthe chamber is large compared to the cross-section of the duct.

Medium-frequency sound waves having wave lengths of the same order ofmagnitude as the transverse dimension of the chamber will to some degreebe reflected from the cross-sectional transition but not to so high adegree as is the case with low-frequency waves. The degree of reflectionwill increase with the proportion of the cross-sections of the chamberand the duct,

although this tendency is hardly as pronounced as is the case withlow-frequency waves.

It may be concluded that in the conventional expansion chamber silencerthe chamber is exposed to a powerful noise from the inlet duct(dependent on the noise source, eg an internal combustion engine) in thefull audible frequency range, and the conditions at the cross-sectionaltransition between duct and chamber shifts this load in the direction ofmedium and particularly high frequencies. The capability of the silencerof reducing the arriving noise is good within the lowfrequency range,but less good within the mediumfrequency range, and within thehigh-frequency range the silencer is ineffective.

Where there is a need for a perceptible reduction of noise within thewhole range from low to high frequencies it has been customary tocombine the expansion chamber silencer with an extra silencer of theadsorption type, such a silencer being effective with regard to highfrequencies, less effective in the case of medium frequencies andineffective at low frequencies. A combination like that mentioned abovemay in a way be excellent, but there is an evident need for an expansionchamber silencer which without the aid of an extra silencer offers asatisfactory reduction of noise within the full range from low to highfrequencies.

in a modification of the conventional expansion chamber silencer theinlet duct is elongated so as to extend a greater or smaller distanceinto the chamber. This does not result in any noticeable change in thehigh transmission of noise to the chamber, but some change is achievedin the capability of the silencer of reducing the medium-frequencynoise. On the assump tion that the chamber is of a regular shape, manyof the natural pulsations of the fluid in the chamber will have theirpressure pulsation node in the centre of the chamber. Consequently, ifthe duct is extended to this centre, such natural pulsations will,principally, not be initiated or struck, and by this means the fluid inthe chamber and thus also the fluid in the outlet duct will be relievedof the energy of sound pulsation corresponding thereto. However, unlessthe length and the transverse dimension of the chamber aredisproportionately large, this effect will be limited to the lowestnatural frequency of the fluid in the chamber, due to the fact that thetransmission of noise from the inlet duct to the chamber takes place notat a point, but over a certain range of volume, the minimum dimension ofwhich corresponds fairly well to the transverse dimension of the duct.By this modification is achieved that approximately a singlemedium-frequency resonance is avoided, whereas a noticeable change inthe noise conditions within the low-frequency and highfrequency rangescannot be expected. The result of the extension of the duct is that thegas jet introduced is localized closer to the hole edge of the outletduct, and this may easiiy produce undesirable edge sounds resulting inan increased high-frequency noise. The gas jet will even have theopportunity of striking natural pulsations in the outlet duct with theresult that the mediumfrequency reduction is reduced. This modificationshould thus be applied suitably carefully, and, at best, the result is acomparatively limited improvement of the medium-frequency reduction ofthe silencer.

In another known modification of the conventional expansion chambersilencer the chamber wall is completely or partially covered by a liningof porous,

sound-absorbing material, eg. mineral wool. ln principle, this measureprovides a reduction of the noise of the fluid in the chamber but thisreduction is only of substantial importance as far as high frequenciesare concerned, because the known absorption materials possess a poorercapability of absorbing sound of medium frequency, let alone sound oflow frequency. However, this modification seems to be of only limitedusefulness, since a better total high-frequency reduction can beachieved by an analogous lining inside the outlet duct. In fact, thereduction of noise measured in db. per unit of length of chamber or ductis approximately inversely proportional to the transverse dimension ofthe flow which is significantly larger in the chamber than in the outletduct.

In a third variant of the known expansion chamber silencer the inletduct is elongated so as to extend a suitable distance into the chamberand is closed by a terminating transverse plate, To permit flowcommunication from the duct to the chamber, the wall of the ductextension is provided with holes made by perforation or slitting. Inthis way the transmission of noise from the duct to the chamber isreduced because the terminating transverse wall is effective to reflectthe arriving sound waves. Now, the transmission of noise occurs throughthe holes in the wall of the duct extension against a resistance that,according to the circumstances, is of a resistive nature, therebyinfluencing low-frequency noise, or of reactive nature, influencinghigh-frequency noise. As, however, the total flowpassage area of theholes should normally be somewhat larger than the duct cross-sectionproper in order that the loss in pressure via the silencer should notbecome too great, the acoutstic resistance of the holes will accordingto experience in reality be modest. A difficulty encountered is,incidentally, that the holes should resist the transmission not only ofthe waves arriving directly from the inlet duct, but also of the soundwaves reflected undiminished from the transverse plate. To this must beadded that the relatively sharp edges of the holes may give rise tohigh-frequency edge sounds. The best result to be achieved isconsequently only a moderate reduction of the transmission of noise fromduct to chamber. In the case of a less favourable shaping of the holesthe fluid in the chamber may be exposed to an increased admission ofnoise, in particular highfrequency noise, dependent on the presence andintensity of edge sounds. An additional drawbach is that, in spite of anample dimensioning, the holes in the wall offer a higher flow resistancethan a duct extension without a transverse plate.

The aim of the invention is to provide a gaseous fluid expansion chambersilencer so designed that the entire content of audible noise in theflow of fluid originating from the noise source, e.g. the exhaustprocess of an internal combustion engine, is reduced essentially by thepassage of the fluid through the silencer, and that this passage isconnected with a particularly low flow resis tance as well as minimum ofturbulence noise.

More specifically, the invention relates to a silencer which is intendedfor the reduction of the transmission of noise by the flow of gaseousfluids through tubular ducts, particularly exhaust pipes for internalcombustion engines, and comprising an expansion chamber casing withapertures for inlet and outlet pipes con' nected thereto, the inlet pipebeing extended into the casing and permitting a lateral flow of fluid tothe casing. According to the invention the silencer is characterized inthat the end of the inlet pipe extension is formed by a preferablysubstantially axis-symmetrical cross plate diffuser, through which theflow of fluid is led to the casing substantially in the form of a thincurtain.

The invention is based on the recognition that the cross plate diffuserpossesses properties that are acoustically favourable in severalrespects. As will be known, such a diffuser consists substantially oftwo plates, one of which is a flowdeflecting full plate while the otheris a hole plate which is substantially parallel to the full plate and isconnected to the inlet duct of the diffuser. To counteract unnecessaryflow losses, the hole edge portion of the plate may be designed with asuitable axis-symmetrical rounding, and the full plate in the followingcalled the cross plate is secured in relation to the hole plate in asuitable way.

Due to its orientation perpendicular to the axial direction of the inletpipe and the pipe extension, the cross plate will to an essential degreereflect highfrequency noise, that is to say will throw the content ofhighly pitched sounds in the flow of fluid back into the inlet duct,since high-frequency sound waves are of a distinctly direction-stableand therefore, in the present case, axis-parallel nature in propagationas well as in reflection. Already for this reason a considerable reduction of the transmission of high-frequency noise from the inlet pipeto the casing is achieved.

The external transverse dimension of the two plates of the diffuser isnecessarily larger than the transverse dimension of the pipe extensionand, consequently, the two plates form together an annular slot-shapedoutlet duct. The mass of the flowing fluid in this outlet slotcontributes inertially to an essential resistance against the passage ofhigh-frequency and medium-frequency noise, and thereby assists the crossplate to reflect such noise components to the inlet pipe.

The narrow outlet slot of the diffuser ensures that the discharge of thefluid into the casing has the nature of a correspondingly narrow andthereby curtain-shaped gas jet. The issue of high-frequency andmediumfrequency turbulence noise from this gas jet will be essentiallysmaller than from the gas jet which the same flow of fluid would form inthe casing if the cross plate diffuser were removed. This is due to thefact that the volume of the flow-turbulent mixing zone of the diffuserwill be reduced to about one third and that the energy density ofhigh-frequency and medium-frequency noise within this volume isessentially smaller in the case of the diffuser jet due to the lowerparticle velocity, caused by the diffuser effect, in the outletaperture. This circumstance also contributes to the fact that when across plate diffuser according to the invention is used, a considerablereduction of the transmission of high-frequency and medium-frequencynoise from the inlet pipe to the casing is achieved.

The good effect of the silencer in the case of lowfrequency noise isensured in a conventional way by a suitable dimensioning of the casingand the associated outlet pipe.

To this must be added that the cross plate diffuser according to theinvention without any difficulty, and while preserving its good effectwith regard to noise suppression, can be designed in such a way that thedrop in pressure from the inlet pipe to the casing can be kept very low,that is to say at a small fraction of the dynamic pressure in the flowof fluid in the inlet pipe. This contributes to keeping the total flowresistance of the silencer at a low level, and this is of importance inmany cases, in particular if the flowing fluid is exhaust gas from aninternal combustion engine, and most certainly if such an engine isprovided with an exhaustoperated turbo-charger.

According to the invention the inlet pipe extension may have an axiallyvarying crosssection before the cross plate diffuser, preferably of aconically converging or a conically diverging nature. This form ischosen with a view to the fact that when a silencer is to be designed orselected for a given purpose it may be desirable to weigh the pros andcons of the desire of the reduction of noise compared to demands as toflow resistance, space requirements etc. Per se, the detailed design ofthe cross plate diffuser will to a wide extent allow such a weighing ofdesires and demands, but the possibilities will be greater if the shapeof the inlet pipe extension can be varied as described above. Aconically convergent pipe member before the diffuser will generally givethe silencer an increased noise reduction and at the same time anincreased flow resistance, while a conically divergent pipe memberbefore the diffuser will show opposite effects.

In a preferred embodiment of the invention, the silencer as a whole isaxis-symmetrical and comprises an expansion casing having acircular-cylindrical shell and two axis-symmetrical end plates, each ofwhich is provided with a central aperture for inlet and outlet pipes,respectively, the extension of the inlet pipe as well as the cross platediffuser being placed coaxially in the casing. The expedient feature ofthis embodiment with a distinct longitudinal direction coinciding withthe axis of symmetry of the silencer is, with regard to the effectachieved, particularly based on the following two circumstances.

Firstly, many of the possible natural pulsations of the fluid in thecasing will be of a longitudinal nature, and this implies that when theaxial positioning of the cross plate diffuser in relation to the casingis chosen it is possible selectively to influence the formation andsuppression of such pulsations. In this connection it is of particularimportance that the flow of fluid from the diffuser has a substantiallyradial direction and a curtain-like and thereby narrow and well-definedgeometrical shape. If the diffuser is positioned approximately halfwaybetween the two end plates ofthe casing, many possible longitudinalnatural pulsations are not struck since, principally, one half of thesepulsations have pressure pulsation nodes in or close to the centraltransverse plane between the end plates, that is to say in or close tothe plane of the curtain flow. Thus, the formation of noise ofsubstantially medium-frequency in the fluid in the casing is avoided.

Secondly, it has turned out that by the curtain flow sweeping the insideof the wall of the casing it is possible, in spite of energy lossescaused by shocks, to achieve a limited, but not insignificantrecuperation of static pressure. The probable explanation of this rathersurprising effect may be that in relation to the curtain flow the wallof the casing acts almost as an extra cross plate diffuser having onlyone flow-conducting plate. Experiments have proved that by a radialdistance between the periphery of the cross plate diffuser and theinside of the wall of the casing of the order of magnitude of 5 timesthe slot width of the diffuser duct, in

combination with an otherwise expedeint design of the cross platediffuser, it is possible to achieve so large an extra pressurerecuperation in the casing that the total pressure loss from inlet pipeto casing equals nil or is even negative.

The inlet pipe extension may be surrounded by sound-absorbing materialwhich transversely to the pipe is spaced from the walls of the expansioncasing. The purpose of the sound-absorbing material is to reduce thecontent of particularly highfrequency noise of the fluid in the casing,and the material may be of a known type, e.g. loose mineral wool whichin a conventional way may be secured within a surrounding jacket ofperforated sheet material.

The pipe extension offers the particular possiblity that the body ofabsorption material may be a stack of pressed mineral wool plates of aknown type, a hole being cut out in each plate for a relatively tightfit on the pipe. Such a plate stack having any suitable surface shapemay easily be fitted on the pipe extension between the hole plate of thediffuser and the wall of the casing with a suitable degree of axialpressure for achieving a good securing and stability of shape.

It may be pointed out that the pipe extension of the silencer shouldhave no perforation or slitting, since this may entail an essential riskof undesirable and loud edge sounds in the flow of fluid.

The two flow-limiting plates of the cross plate diffuser may beinterconnected by means of intermediate ribs which are preferably radialand have a rounded front edge facing the flow of fluid. The ribs serveprimarily for securing the cross plate to the hole plate, so that theirmutual location and consequently the flow geometry of the diffuser arepreserved under all circumstances. This embodiment of the diffuser willbe suited for being produced by diecasting or by sheet metal work. Thelength, cross-sectional shape and location of the ribs may be chosen atwill ifonly the requirement as to securing is fulfilled, but the ribsmust not, however, essentially detract from the properties of thediffuser regarding noise and flow. It is of particular importance thatthe front edge of the ribs facing the flow of fluid is rounded tocounteract the formation of edge sound. With a view to this it may beexpedient if the front edge of the ribs has been extended in front ofthe hole edge of the hole plate, since edge sounds, if any, will then bereflected from the cross plate of the diffuser together with otherhigh-frequency noises in the arriving fluid. The extended portion of theribs may moreover serve as centering guide when the diffuser is beingassembled with the extension of the inlet pipe.

In another embodiment of the silencer the two flowlimiting plates of thecross plate diffuser are secured to each other at their periphery bymeans of preferably radially oriented ribs with a roundedcross-sectional form of the inner edge facing the flow of fluid, and thetwo plates are furthermore by means of the same ribs rigidly connectedto the wall or end plates of the expansion casing. This embodiment issuited for being produced by sheet-metal work and welding. In this casethe ribs contribute to securing the diffuser firmly in the easing, andthis may be necessary in cases when the silencer is exposed tovibrations and shocks.

A special embodiment of the silencer is characterized in that the ribsare coaxial with the cross plate diffuser and the extension of the inletpipe and form stay elements that are connected to the wall of theexpansion casing at the aperture of the outlet pipe so as to beaxissymmetrical around this aperture. In the vicinity of this aperturethe ribs furthermore form fastening means for a shield which togetherwith the casing wall around the aperture bounds on outlet passage forthe fluid flow leading to the aperture. The space within the ribs andbetween the shield and the cross plate diffuser is preferably completelyor partially filled with sound-absorbin g material.

This embodiment of the silencer is of particular interest in the casewhere the silencer as a whole is axissymmetrical, and especially whenabsorption material, consisting of circular discs of pressed mineralwool plate and having an outer diameter corresponding to the outerdiameter of the diffuser is positioned around the inlet pipe extensionas well as in the space between diffuser and shield. By this means avery effective reduction of the high-frequency noise in the fluid in thecasing can be achieved. If the diffuser is located halfway between theend plates of the casing, and if the radial distance from the peripheryof the diffuser to the wall of the casing is about 5 times the width ofthe diffuser slot, and if an outlet pipe of a sufficient length is used,a silencer is provided which with a minimum of flow resistance and spacerequirements to an essential degree reduces all audible noise in theflow of fluid.

The invention will now be more fully explained with reference to theaccompanying drawings, in which FIG. 1 is a vertical section taken online H in FIG. 2 in a first embodiment in which the silencer casingconsists of a cylindrical, vertical shell with domed end plates andradially directed inlet and outlet pipe branches,

FIG. 2 a horizontal section taken on line Il-Il in FIG. 1,

FIG. 2A a section of FIG. 2 on a larger scale,

FIG. 3 is a vertical section taken on line IIIIII in FIG. I in the crossplate diffuser of the silencer,

FIG. 4 an axial section in another embodiment of the silencer with axialfluid passage, and

FIG. 5 a cross-section taken on line V-V in FIG. 4

The silencer shown in FIGS. 1-3 comprises an expansion casing consistingof a cylindrical shell 1 and two outwardly domed end plates 2 and 2'. Atdifferent levels the shell 1 has apertures 3 and 4 for a radial inletpipe 5 and a radial outlet pipe 6. These pipes are outside the casingshown as short pipe branches to which pipe members can be connectedhaving dimensions suited to the conditions. The inlet pipe 5 has anextension 5' which in the embodiment shown is cylindrical as far as tothe axis of the casing and after that consists of a conically divergingsection 5", the wide end edge of which is by welding connected smoothlyto the inner edge of a circular hole plate 7 which is located at rightangles to the axis of the inlet pipe and the inner edge portion of whichforms a smooth transition rounding from the pipe section 5". Parallel tothe hole plate 7 and at a slight distance therefrom is mounted acircular full plate 8 which according to FIGS. 1-3 is connected to thehole plate 7 by means of three radial ribs 9, the height of whichdetermines the width of the annular slot I between the two plates 7 and8 and the inner end portions 11 of which are interconnected and extend ashort distance into the pipe section where they terminate with concavefront edges 12 and the cross-section of which is rounded towards theflow of fluid, see FIG. 2A.

The extension 5', 5" of the inlet pipe is surrounded by porous,sound-absorbing material 13 which fills the space from the shell 1 tothe hole plate 7, but has a free surface 14 transversely to the pipeextension. As shown, this sound-absorbing material may expedientlyconsist of a stack of hole discs 15 which fit suitably tightly aroundthe pipe extension 5', 5", and which can be deformed, FIG. 2, so as tofill the said space completely.

ln FIGS. 4 and 5 there have for corresponding or analogous componentsbeen used the same reference numerals as in FIGS. l3. The silenceraccording to FIGS. 4 and 5 comprises a casing consisting of a shell 1and two conical end plates 2 and 2. An inlet pipe 5 extends centrallythrough the end plate 2 and is provided with a cylindrical extension 5',which in the zone at the axial centre of the casing is assembled with ahole plate 7, the edge portion of which is secured axially and radiallyby engagement with recesses in the inner edge of four equidistant ribs20 which are spaced slightly from the inside of the shell 1 and areparallel to the axis of the latter and are supported by the end plate 2with the outlet pipe 6 of the silencer. A full plate 8 is securedcorrespondingly and in such a position that together with the hole plate7 it forms an annular outlet slot 10.

At a comparatively short distance from the end plate 2' a shield 21which bounds an outlet passage 22 is fitted in the space between theribs 20. Between the shield 21 and the full plate 8 a sound-dampingmateial 23 is inserted which may be of the same type as thesound-damping material 13 around the extension 5' of the inlet pipe.

For the sake of completeness should be mentioned that one and the samesilencer casing may comprise two or more inlet pipes, eg one from eachcylinder, and possible also several outlet pipes, and that severalsilencers may be combined in series with suitably dimensionedcommunication ducts which serve as inlet and outlet pipes for thedifferent silencer casings. The communication ducts need not berectilinear and this together with the small space required by eachsilence will in many cases make possible a convenient mounting of thecomplete silencer unit, eg in the undercarriage of buses or lorries.

The silencer according to the invention is also well suited inconnection with suction plants, eg for the renewal of air in livingrooms, or as a suction silencer for internal combustion engines andcompressors.

I claim:

I. A silencer for gaseous fluid flows including a casing forming anexpansion chamber, inlet and outlet pipes communicating with saidchamber into which one end of said inlet pipe is extended, and across-plate dif fusor comprising an apertured plate flaring from the endof said inlet pipe and forming a generally curved extension thereof, andan unapertured plate spaced axially from and extending substantiallyparallel to the outer portion of said apertured plate, the outerportions of said plates being spaced radially from a wall of said casingand forming together a slot through which the fluid flow from the inletpipe enters said expansion chamber as a curtain flow laterally to theflow direction in said inlet pipe, the average radius of curvature ofsaid extension being at least as great as the width of said slot.

2. A silencer as claimed in claim I, wherein the radial spacing of saidouter plate portions spaced radially from the wall of said casing is atleast as great as the width of said slot.

3. A silencer as claimed in claim 1, wherein said extended inlet pipeend has a gradually increasing crosssection upstream of said aperturedplate.

4. A silencer as claimed in claim 1, wherein said casing comprises aregularly cylindrical peripheral wall and a pair of axis-symmetrical endwalls having central apertures for the inlet and outlet pipe,respectively. said extended inlet pipe end as well as said cross-platediffusor being located axially in said casing.

5. A silencer as claimed in claim 1, wherein said generally curvedextension has a constant radius of curvature.

6. A silencer as claimed in claim 1, wherein said inlet pipe endextending into said chamber is surrounded by a sound-absorbing materialhaving an outer peripheral surface that is spaced radially from saidwall.

7. A silencer for gaseous fluid flows including a casing forming anexpansion chamber, inlet and outlet pipes communicating with saidchamber into which one end of said inlet pipe is extended, a cross-platediffusor comprising an apertured plate flaring from the end of saidinlet pipe and forming a generally curved extension thereof and anunapertured plate spaced axially from and extending substantiallyparallel to the outer portion of said apertured plate, the outerportions of said plates being spaced radially from a wall of said casingand forming together a slot through which the fluid flow from the inletpipe enters said expansion chamber as a curtain flow laterally to theflow direction in said inlet pipe, the average radius of curvature ofsaid extension being at least as great as the width of said slot, andmeans rigidly interconnecting said plates.

8. A silencer as claimed in claim 7, wherein said plates areinterconnected by means of intermediate ribs extending outwardly fromsaid inlet pipe and having rounded front edges facing the flow in saidinlet pipe.

9. A silencer as claimed in claim 7, wherein said plates areinterconnected by means of a plurality of ribs extending substantiallyparallel to said inlet pipe and secured to said casing and to saidplates at the periphery thereof.

10. A silencer as claimed in claim 9, wherein said ribs at one endthereof are secured to a wall of said casing at points lying on a circlesurrounding said outlet pipe, a shield carried by said ribs and formingtogether with said wall a fluid outlet passage leading to said outletpipe, said shield being axially spaced from said unapertured plate, andsoundabsorbing material in the space between the shield and saidunapertured plate.

11. A silencer comprising a casing, an inlet pipe and an outlet pipe,means causing inlet gas to flow longitudinally within said casing in afirst portion thereof, means including two parallel plates transverse tothe longitudinal direction of the silencer for converting longitudinalflow to a radial planar flow including a first said plate connected tosaid inlet pipe, said connection being smooth and continuously curved,said second plate comprising a non-perforated barrier to gas flow andbeing disposed centrally within said casing with a peripheral openingtherearound, the outer portions of said plates being spaced from saidcasing and forming together a slot through which the fluid flow fromsaid inlet pipe enters the chamber within said casing as a curtain flow,and cylindrical passage means downstream from said second plate forconverting radially moving gases to generally longitudinal movementtoward said outlet pipe, said plates being mutually proximate to affecta primary portion of a conversion from gas velocity to smoothes gaspressure.

1. A silencer for gaseous fluid flows including a casing forming an expansion chamber, inlet and outlet pipes communicating with said chamber into which one end of said inlet pipe is extended, and a cross-plate diffusor comprising an apertured plate flaring from the end of said inlet pipe and forming a generally curved extension thereof, and an unapertured plate spaced axially from and extending substantially parallel to the outer portion of said apertured plate, the outer portions of said plates being spaced radially from a wall of said casing and forming together a slot through which the fluid flow from the inlet pipe enters said expansion chamber as a curtain flow laterally to the flow direction in said inlet pipe, the average radius of curvature of said extension being at least as great as the width of said slot.
 2. A silencer as claimed in claim 1, wherein the radial spacing of said outer plate portions spaced radially from the wall of said casing is at least as great as the width of said slot.
 3. A silencer as claimed in claim 1, wherein said extended inlet pipe end has a gradually increasing cross-section upstream of said apertured plate.
 4. A silencer as claimed in claim 1, wherein said casing comprises a regularly cylindrical peripheral wall and a pair of axis-symmetrical end walls having central apertures for the inlet and outlet pipe, respectively, said extended inlet pipe end as well as said cross-plate diffusor being located axially in said casing.
 5. A silencer as claimed in claim 1, wherein said generally curved extension has a constant radius of curvature.
 6. A silencer as claimed in claim 1, wherein said inlet pipe end extending into said chamber is surrounded by a sound-absorbing material having an outer peripheral surface that is spaced radially from said wall.
 7. A silencer for gaseous fluid flows including a casing forming an expansion chamber, inlet and outlet pipes communicating with said chamber into which one end of said inlet pipe is extended, a cross-plate diffusor comprising an apertured plate flaring from the end of said inlet pipe and forming a generally curved extension thereof and an unapertured plate spaced axially from and extending substantially parallel to the outer portion of said apertured plate, the outer portions of said plates being spaced radially from a wall of said casing and forming together a slot through which the fluid flow from the inlet pipe enters said expansion chamber as a curtain flow laterally to the flow direction in said inlet pipe, the average radius of curvature of said extension being at least as great as the width of said slot, and means rigidly interconnecting said plates.
 8. A silencer as claimed in claim 7, wherein said plates are interconnected by means of intermediate ribs extending outwardly from said inlet pipe and having rounded front edges facing the flow in said inlet pipe.
 9. A silencer as claimed in claim 7, wherein said plates are interconnected by means of a plurality of ribs extending substantially parallel to said inlet pipe and secured to said casing and to said plates at the periphery thereof.
 10. A silencer as claimed in claim 9, wherein said ribs at one end thereof are secured to a wall of said casing at points lying on a circle surrounding said outlet pipe, a shield carried by said ribs and forming together with said wall a fluid outlet passage leading to said outlet pipe, said shield being axially spaced from said unapertured plate, and soundabsorbing material in the space between the shield and said unapertured plate.
 11. A silencer comprisinG a casing, an inlet pipe and an outlet pipe, means causing inlet gas to flow longitudinally within said casing in a first portion thereof, means including two parallel plates transverse to the longitudinal direction of the silencer for converting longitudinal flow to a radial planar flow including a first said plate connected to said inlet pipe, said connection being smooth and continuously curved, said second plate comprising a non-perforated barrier to gas flow and being disposed centrally within said casing with a peripheral opening therearound, the outer portions of said plates being spaced from said casing and forming together a slot through which the fluid flow from said inlet pipe enters the chamber within said casing as a curtain flow, and cylindrical passage means downstream from said second plate for converting radially moving gases to generally longitudinal movement toward said outlet pipe, said plates being mutually proximate to affect a primary portion of a conversion from gas velocity to smoothes gas pressure. 